Incremental printing with liquid-base colorants is subject to several very subtle but undesired image defects. Although these arise in generally understood ways from operation of the associated mechanical components, they have nonetheless been very resistant to corrective action.
Many forms of incremental printing operate by creating inkdrop swaths. These swaths are formed in successively stepped positions, by iterated relative motion—along a print-medium advance direction—between the medium and an inking device. Such swath-based printing systems may be of a scanning type, operating by repeated operation of the inking devices across the medium, or a pagewide swath-height array type.
Some artifacts, even though due to different apparatus phenomena, are often designated by the generic term “banding”—meaning that they usually present the appearance of subtle bands, stripes or striations. This is particularly true for swath-based devices, or scanning devices—although artifacts of interest with respect to preferred embodiments of the invention are not limited to such systems.
The reason for this commonality with respect to banding artifacts is that almost any cyclical or near-cyclical perturbation of inking operations creates some form of correspondingly periodic visible pattern. Merely by way of example, features that appear in generally the same elevation across each swath naturally tend to produce a visual effect at the spatial periodicity of the printing-medium advance.
When printing with a single layer of swaths fitted edge-to-edge, that periodicity is in principle equal to the swath height. (Such an edge-to-edge arrangement is obtained through use of a so-called single-pass printing mode, for a scanning system—or more generally a single-inking-installment mode, to encompass a pagewide swath system.)
In plural-installment modes—i.e., with overlapping swaths—the spatial periodicity of the banding is finer. In these cases the print medium usually advances by some fraction of the swath height, giving rise to the finer banding.
Some types of banding—and, more generally, artifacts addressed by some forms of the present invention—are not fundamentally swath-related at all. Again merely by way of example are those artifacts arising from tiled relatively small printmasks (discussed below).
Such artifacts are most conspicuous in midtone regions of an image, where there is modulation range in either direction to exhibit extremely subtle patterns. Other artifact types, for example those associated with slight overinking effects (also discussed below), are instead most conspicuous in darker regions where colorant liquid effects can dominate.
(a) Boundary banding—One of the major contributors to banding is a thin, darker line that appears along one or usually all edges of a printed field. This is the so-called “boundary banding”; however, more generally it should be regarded as a boundary artifact.
In a swath-based system, some manifestations of this type of artifact appear to be concentrated particularly where two swath edges abut, or nearly abut. Boundary artifacts are very hard to remove—especially when printing in swaths, and especially at a low number of passes or installments.
Normally they appear only in image regions that are rather highly saturated calorimetrically. Such colorimetric saturation giving rise to boundary artifacts, however, occurs in either:                a single primary colorant (ordinarily cyan, magenta, yellow and black), or        a composite color formed from combinations of those colorants in various proportions.Accordingly this localization of the banding is believed to be a liquid-loading effect—coalescence of the liquid in adjacent inkdrops, concentrated by surface tension at the edges of the just-deposited liquid field.        
Boundary artifacts are hard to attack when associated with heavy inking in one primary. This difficulty is exacerbated when the overinking takes the form of an aggregation, with only modest contributions from plural colorants as in the case of composite colors.
Another form of banding—along horizontal boundaries only—arises from swath-height error (“SHE”, and sometimes “SWE”) rather than coalescence. It will be discussed below.
(b) Other swath-associated artifacts—One distinctly different kind of image defect, although it too strongly affects contiguous-swath abutments, relates to swath-height error. This type of error usually occurs when nominal relationships between swath height 135 (FIG. 1A) and printing-medium advance 133 fail.
In the nominal relationship, when the effective pen height 135 just matches the advance distance 133, swaths 131, 132 abut neatly 134. For nominal advance, such relationships are maintained when ink-discharging nozzles near the inking-array edge are pointed straight toward the printing medium 130 along a normal to the surface.
When those end nozzles instead point outboard or inboard—along the print-medium advance direction—such misdirections cause the swath to be taller 135′ or shallower 135″, respectively, than its nominal height 135. In the former case, for a nominal print-medium advance stroke 133, excess lengths 136, 137 at top and bottom, respectively, of overlong adjacent swaths 131′, 132′ then overlap slightly. The overlap forms a dark line 134′ (FIG. 1B) along the swath boundary.
In the opposite case of inboard-pointing end nozzles causing shallower swaths 135″, the foreshortened regions 138, 139 at top and bottom, respectively, of the undersize adjacent swaths 131″, 132″ fail to abut at all. The failure 134″ to abut, leaves a white line 134″ (FIG. 1C) between the swaths.
In practice, these conditions arise also with nominal swath height, for short stroke and long stroke respectively. (If the only problem is inaccurate stroke, however, then correction is straightforward and easy.)
Through fine adjustment of the advance stroke, this kind of mismatch between stroke and swath height can be hidden, for some one particular magnitude of nozzle-direction error, but not entirely cured. The previously introduced patent documents of Cluet, Donovan and Doval, Vilanova '499 and particularly Subirada '652, introduce various techniques for attacking this problem—and other errors related to nozzle health, taken up shortly.
Generally speaking such techniques also require printing and measurement of test patterns designed to reveal details of the banding characteristics to be overcome. Measurements of this sort are facilitated by apparatus and methodology introduced in several related patent documents, particularly those of Baker, Bockman, Borrell (serial '858), Soler, Subirada and Vilanova '207—as well as the three others noted in the preceding paragraph.
Unfortunately, however, stroke adjustment either foreshortens or lengthens, respectively, the overall image. The image is typically made a significant fraction of one percent too tall or short.
Worse, such a corrective tactic cannot restore image detail that is either blurred or lost, respectively, as the abutment region is distorted in height relative to areas within the swath. Even worse yet is the inability of such strategies to accommodate more than just one particular magnitude of directional error, when in fact each color in an image is printed from a respective different inking array—i.e., printhead, or so-called “pen”.
For instance suppose that in a particular printer one of these pens prints, in one color, a swath 131″ that is 0.3% too shallow 135″—while another pen makes, in a second color, a swath 131′ that is 0.4% too tall 135′. These two arrays, and their respective two color swaths 131″, 131′, are intrinsically out of register with each other by approximately 0.7%. Any attempt to adjust the stroke 133 to hide swath-height lengthening error 136, 137 or foreshortening error 138, 139 in either of the two colors must necessarily worsen the effect for the other.
In a modern system there are at least four pens. The likelihood of significant mismatch between two is accordingly sizable. Although in principle pens can be sorted into matched sets, doing so increases cost—and in any event the ink is generally exhausted from one pen faster than another, so that the practical usefulness of such an approach is limited.
These problems are particularly severe in single-pass (single-installment) printing modes. For instance abutment failure 134″ between two swaths in a particular color leaves an unprinted strip all the way across the image.
If other colors happen to be unused in that region of the image, that unprinted strip is white. The effect is often very conspicuous even if other colors are present, especially since the unprinted strip repeats at intervals equal to the advance stroke. Like boundary artifacts, this type of error is maximally obtrusive when occurring in a highly saturated field of a dark color.
Another type of artifact can arise from an error type that is related to swath-height error: nozzle pointing errors of uncorrelated magnitude and sign, in the advance axis but within the swath rather than at the ends. As adjacent nozzles in different segments along a nozzle column can point either toward or away from each other, results typically include both underinked and overinked strips, respectively.
Thus, like swath-height error but unlike boundary artifacts, these internal pointing errors can create faded or unprinted zones as well as overly dark zones. Unlike both the boundary artifacts and swath-height errors discussed above, these pointing errors naturally are not localized at swath boundaries.
Further, these artifacts may be either isolated from one another or closely grouped, depending entirely on all the conditions of the nozzle array. The entire striation pattern, however, as with those two above-discussed errors, does repeat at intervals equal to the medium-advance stroke.
Still another error source is somewhat related to internally misdirected nozzles: incorrect inkdrop size, or an extreme case of it—total nozzle failure. Inkdrop size can vary due, for example, to low or high firing energy, or to the mechanical characteristics of a heater resistor or a nozzle as manufactured, or to plugging or other degradation of a nozzle through use.
Resulting striations, like those due to internal pointing error, can be either light or dark. They can also be either isolated or clustered within a swath.
Banding artifacts due to swath-height errors, internal pointing errors and inkdrop size errors alike may be classed as “area fill nonuniformity”. Such nonuniformity, observed in printmodes with enough passes to conceal boundary artifacts, is mainly generated by a combination of various swath-height errors.
Heretofore a primary strategy for reducing area-fill nonuniformity is adjustment of the advance stroke. The strategy includes selecting an optimum stroke that maximizes image quality. Each pen, however, has its own, different nozzle profile—ideally leading to a specific stroke value for that particular pen.
Under these circumstances, precise compensation of all the pens with just one advance is not possible. The best that can be done is a compromise, and obtaining an ideal compromise requires an advanced and somewhat elaborate procedure.
Such a procedure takes into account characteristics of the printing medium as well as the banding appearance. The procedure also incorporates decisional algorithms to determine the optimized compromise advance for each swath. These requirements also in effect build another kind of compromise between image-quality improvement and throughput loss.
(c) Swath-independent over- and underinking—These topics relate to excess or inadequate inking that is not localized with respect to a swath—but rather only arises through relatively extreme color-saturation requirements in an image. Accordingly these problems, as will be seen, are only tangentially related to this document.
To achieve vivid colors in printing with liquid inks, and to substantially fill the white space between addressable pixel locations, ample quantities of ink must be deposited. Doing so, however, requires subsequent removal of the liquid base—by evaporation (and, for some printing media, absorption)—and this drying step can be unduly time consuming.
In addition, if a large amount of ink is put down all at substantially the same time, within each section of an image, related adverse bulk-colorant effects arise: so-called “bleed” of one color into another (particularly noticeable at color boundaries that should be sharp), “blocking” or offset of colorant in one printed image onto the back of an adjacent sheet with consequent sticking of the two sheets together (or of one sheet to pieces of the apparatus or to slipcovers used to protect the imaged sheet), and “cockle” or puckering of the printing medium.
In a sense these excess-liquid problems arise because colorant quantities are determined from colors specified in an image file, which are developed without regard for inking—especially for aggregate liquid—needed to implement those colors. Such color specifications are created by artists, or derived from photographs or other preexisting images, none of which takes into the account the liquid loading associated with all colorants in the aggregate.
Various techniques are known for use together to moderate these adverse drying-time effects and bulk- or gross-colorant effects. It is helpful to bear in mind, however, that the overall total amount of ink in a region should be actually reduced only as a last resort, since all this ink is what is appropriate for the desired color.
An opposite sort of problem arises when geometrical relationships between ink dots and pixels prevent attainment of linearity in actual color saturation—even though nominally full inking is specified by an image file. This phenomenon, as described at length in the Borrell serial '163 document mentioned earlier, results in inadequate apparent visual saturation of colors in image areas that are fully inked.
Once again this particular form of underinking is not at all localized with respect to printheads or swaths. Rather, it transcends such mechanical phenomena, and relates strictly to image color considerations.
(d) Depletion and propletion—The excess-liquid deposition described above is managed by a process called “depletion”, long a familiar one in inkjet printing. This process pauses to correct the absence of aggregate-liquid accounting in the original development of color specifications for an image.
Thus the depletion process typically includes adding the numbers of drops of all colorants at a pixel, and preferably considering the average of such drop count over some practically determined local area. When the resulting quantity exceeds a threshold established through experience with the printing medium, humidity and such considerations related to drying speed, the process may conclude with modification of the derived inking data.
This modification consists of reducing the drop count, usually in such a way as to exert minimal degradation of color accuracy. This condition, however, is a difficult one—since composite-color hue is very sensitive to colorant proportions, and even the brightness of primary colors is sensitive to the amount of colorant deposited.
Concern for such vividness, or at least its linear response in true visual terms, is at the heart of the Borrell '163 document. Borrell provides for addition of ink—where the averaged liquid loading will permit—when needed to realize the visual effects implicit in image data.
Like the over- and underinking conditions that they are designed to correct, both depletion and propletion are essentially swath independent. That is, they transcend swath structures and are localized only with regard to the image itself.
(e) Printmode techniques—Another useful technique for concealing both banding and excess liquid deposition is laying down in one inking operation by the pen only a fraction of the total ink required in each section of the image. Any artifacts—areas that are either darkened or left unprinted in that inking—are visually diluted by one or more later inking installments.
Consider, for example, a dark strip. After printing of all the installments, although it remains darker than adjoining color fields, it is much closer to them—especially on a fractional or logarithmic basis—and therefore less conspicuous to the human logarithmic visual response. It typically is printed in only a fraction of the installments (in one installment out of, for example, three or eight), while the adjacent areas are printed with all the installments. Similarly an unprinted strip ordinarily lacks only the ink that should be printed in one installment, being filled in for all the others.
This technique is applicable equally to installments performed by firing a pagewide swath-height array and by scanning a small swath-height printhead across the printing medium. Each installment in a pagewide-array system may be called a “shot”, and in a scanning system is usually called a “pass”. For simplicity of expression here, the word pass is sometimes used to refer to both.
Thus operation with a single inking installment for each image region may be called a single-pass mode, and with more than one may be termed a plural-pass mode—or if more than two a multipass mode. The concept of plural-pass printmodes encompasses multipass operation.
The benefits of plural-pass printmodes are not limited to suppressing the conspicuousness of almost all artifacts, by the visual-dilution effect mentioned above. In addition plural-pass modes tend to control bleed, blocking and cockle by reducing the amount of liquid that is all on the page at any given time, and also may facilitate shortening of drying time.
The specific partial-inking pattern employed in each pass, and the way in which these different patterns add up to a single fully inked image, is known as a “printmode”. Heretofore, as reported in several of the patent documents enumerated above—particularly those of Garcia and Gil—great advances have been made in the design and implementation of printmodes as such, and the data pipelines that implement them.
All those refinements are generally outside the scope of the present document. Some related innovations, however, will be discussed later in this document.
What is particularly important for present purposes is that printmasking cannot cure either problem—either overinking or banding—in its entirety. Excess-liquid problems reassert themselves as printing throughput increases. Banding, although its conspicuousness is very significantly depressed by multipass printmodes, nevertheless—by the nature of the above-discussed dilution mechanisms—does not disappear entirely.
The remaining band structure is often made extremely subtle, but yet visible and annoyingly persistent. This residual effect often stands out in particular image tonal ranges or colors, and becomes more and more important in a competitive market.
Furthermore multipass printmasking itself obstructs throughput increase, which is now highly prized in that same marketplace, and modern systems are trending back toward low-pass-number printmodes. Hence other solutions must be sought for banding problems.
(f) Other techniques—A typical solution for reducing boundary artifacts has historically been to increase the number of passes. This is a trade-off rather than a solution, since printing times are seriously degraded.
Another approach consists of staggering or semistaggering the printheads. Since different printheads do not print their swaths on exactly the same line of pixels, coalescence is not as serious, and the boundary artifacts are much attenuated.
Semistaggering of printheads, though, produces hue-shift banding in bidirectional printing (that is, different portions of the swath show slightly different colors). Fully staggered printheads are not easy to support mechanically, because they require a very wide, flat paper path.
Still another approach, pursued actively in recent years, consists of diminishing the usage of end nozzles of the printhead. Ink is not deposited onto the media as a step function (sudden transition from dry to wet, along the paper-advance axis), but rather is applied as a smoothly growing function—also called a ramp, nozzle ramp, or nozzle tapering.
This technique is good for a relatively high number of passes (more than about four), but remains insufficient for fewer-pass printmodes (one to four passes). The reason is that the work not done by those end nozzles must be compensated by some backup nozzles, and the number of such backup units that are available is squeezed to the vanishing point as the number of passes is decreased.
The printmasking algorithm becomes more and more constrained with fewer passes and therefore fewer backup nozzles. In addition, steep ramps produce “big drop/little drop” effects (already-inked paper and dry paper display drops differently) that show up as hue-shift banding. So eventually, and for a low number of passes, the state of the art leads to trade-off decisions between boundary and hue-shift banding.
(g) Conclusion—Banding artifacts have continued to impede achievement of uniformly excellent inkjet printing at high throughput. Thus important aspects of the technology used in the field of the invention remain amenable to useful refinement.